Methods to improve efficiency of CO2-based refrigerating systems: zeotropic mixture, energy storage, cycle modifications

The Heating, Ventilation, Air Conditioning and Refrigeration (HVAC&R) sector plays an essential role into the 21st-century life, silently but significantly influencing numerous domains, including but not limited to food preservation, indoor climate control, healthcare, industrial processes, and energy management. According to the IIR, it is estimated that there are approximately 5 billion refrigeration, air-conditioning, and heat pump systems currently operational around the globe. Still according to IIR, the refrigeration sector, including air conditioning, consumes about 20% of the overall electricity used worldwide with the demand that could be more than double by 2050. The sector is responsible for 7.8% of the world greenhouse gas emissions (4.14 GtCO2eq), from which 37% are caused by direct escapes of refrigerants and 63% are related to indirect emissions due to use of electricity. This thesis aims to reduce these two sources of environmental impact by presenting a work that involves CO2 as a refrigerant fluid used as a method to reduce direct emissions, with the focus on improving its utilisation as a solution to indirect emissions. The work is structured in two parts, with the first focusing on the possibility of increasing the efficiency of CO2 plants by doping carbon dioxide, and the second focusing on analysing different possible configurations of both the refrigeration cycle and also of the whole plant, in this case with the use of a thermal storage. The order of the chapters follows a structure that focuses on the mixtures at the beginning, enlarging the viewpoint on the different cycles and then further enlarging the viewpoint on the global system connected to the supermarket building. Several CO2-doped blends are evaluated theoretically and then experimentally. Experimental tests show the potential for enhancement of COP in typical CO2 cycles with the use of mixtures replacing pure CO2; in particular, the use of CO2/R-152a [90/10%] and CO2/R-152a [95/5%] mixtures provided maximum COP improvement of 10.2% and 10.6% respectively for the same heat rejection temperature. On the other hand, for a cycle with internal heat exchanger (IHX), the use of the new mixture decreases energy efficiency with the only exception observed with CO2/R-152 [95/5%] and an inlet temperature of 35 °C, which led to a 0.4% improvement in COP. Finally, the use of a non-azeotropic mixture in a dedicated mechanical subcooling (DMS) system improves efficiency by only 0.46% with the R-600/R-152a [60/40%] blend however confirming the theoretical results trend. Moving on to the second part, an analysis of commercial CO2 refrigeration cycles is conducted. Four CO2 cycles were experimentally compared in the same plant over four ambient conditions, with a maximum improvement by 4.64% and 9.47% when ejector and IHX cycles are used respectively. A variable-diameter nozzle and liquid CO2 pump were assessed as ejector control methods; the pump operated stably and was able to increase the ejector efficiency by 11%. Then a real case of a supermarket with an Ice Thermal Energy Storage (ITES) is analysed, where the storage can be fruitfully used to shave peaks in electricity use. In the particular configuration analysed, the storage shows to be detrimental for the energy efficiency; however, the cost analysis shows that the reduction in size of the reversible heat pump, and the chance to avoid the installation of an electrical transformer in a dedicated room allows saving up to 58.699 € in 10 years, thus making the choice of ITES more profitable in the usual lifetime for these plants.